The present invention relates to a coding method and apparatus involving parallel concatenated codes linked by an interleaver, for use in wireless digital transmission.
Parallel concatenated convolutional codes, known as xe2x80x98Turboxe2x80x99 codes, have been disclosed in the paper xe2x80x98Near Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes(1)xe2x80x99, Berrou, Glavieux and Thitimajshima, ICC""93 Geneva, May 23-26, 1993, as well as in U.S. Pat. No. 5,446,747. This type of code has attracted much attention in the digital transmission field because of its bit error rate performance close to the Shannon limit. The Turbo encoder as originally proposed consists of two recursive systematic convolutional coders. The two encoders receive the same information bits, but the input of one of the encoders is connected to an interleaver so that the order of the input bits is scrambled.
Parallel concatenated encoders using constituent codes other than convolutional codes have also been proposed, for example in the article xe2x80x98Unveiling Turbo Codes: Some Results on Parallel Concatenated Coding Schemesxe2x80x99 by Benedetto and Montorsi, IEEE Transactions on Information Theory, Vol. 42, No. 2, March 1996, and in xe2x80x98Iterative Decoding of Turbo Codes and Other Concatenated Codesxe2x80x99, a dissertation dated February 1996, by S. A. Barbulescu of the Institute of Telecommunications Research, University of South Australia.
However, the use of the interleaver in Turbo and related codes results in a long encoding delay, which has prevented their adoption in real time applications such as digital mobile telephony (see the article by Benedetto and Montorsi cited above).
According to the present invention, there is provided a parallel concatenated encoder which generates a sequence of data sets for transmission. Interleaved parity bits are not included in earlier data sets, which contain data bits and non-interleaved parity bits. As a result, the delay incurred by interleaving the interleaved parity bits is not fully incurred in the output of the sequence of data sets as a whole; at the same time, uneven distribution of data bits and parity bits is avoided in a substantial part of the sequence.
In some embodiments of the invention, the interleaved parity bits have an index constraint substantially less than the size of the interleaver. This allows interleaved parity bits to be output before all of the data bits have been stored in the interleaver, further reducing the delay incurred by the encoder, without significantly increasing the bit error rate over a transmission channel.